This invention relates to the torque measurement field, and more particularly to torque measuring methods and apparatus wherein two toothed wheels, disks, gears or the like are attached to a rotating shaft at some distance apart and the relative positions of the teeth are measured as the twist in the shaft operates to change the position of the teeth with respect to each other.
Existing torque measuring devices of this type often have used two sensors, each monitoring the rotation of one of the gears. See, for example, Brown, U.S. Pat. No. 4,136,559. One disadvantage of this arrangement is that the sensors must be kept accurately aligned with respect to each other. Misalignment of the sensors will lead to torque measurement error. An additional disadvantage of this arrangement is the requirement for a great deal of free space around the rotating shaft. In the space restricted environment of most rotating machinery, there may not be enough room to mount two sensors and two spaced apart gears.
To overcome such problems, monopole sensing schemes have been adapted to the torque measuring field. In Parkinson, U.S. Pat. No. 3,548,649, two gears are placed in near adjacency by fixing one of the gears to a reference sleeve which is, in turn, fixed to the shaft at a predetermined distance from the other gear. Thus, Parkinson +649 uses a monopole sensor to detect relative movement of the two gears. However, to do so, Parkinson '649 interlaces the gear teeth and measures the relative spacings therebetween to arrive at torque.
An analogous technique is adopted in Tenkman, U.S. Pat. No. 3,844,168. There, a monopole sensor is also used to detect the passage of teeth. However, Tenkman '168 does not use a sleeve to bring the teeth into near adjacency. Instead Tenkman '168 projects individual teeth down the shaft from their point of attachment to the location of the sensor.
The reference sleeve technique is also used in Spinella, U.S. Pat. No. 3,888,116 and Ward et al, U.S. Pat. No. 3,940,979. These patents disclose adjacent gears straddled by an optical emitter and sensor. Light shining through the gear teeth is used to measure spacing between the teeth of the two gears.
Torque measurement can be made more accurate by the use of vernier measuring techniques wherein one of the gears is supplied with a different number of teeth than the other. Thus, teeth from the two gears will overlap by a varying amount around the periphery of the gears until, at one point, two teeth are in near alignment. As torque operates to twist the shaft, the location where two teeth are in near alignment will move around the periphery of the gears. By measuring the distance from a known trigger point to the location of near tooth alignment, an accurate measurement of torque can be obtained.
Such a vernier technique was generally disclosed in Wilson, U.S. Pat. No. 3,194,065. However, Wilson '065 employs three separate sensors, one for each of the gears and one to detect a separate projecting trigger point.